What would happen if someone jumped in the front of a Google car?
Given this YouTube video which is being given by Sebastian Thrun who had a TED talk which had nowhere near the same level of detail but had similar conclusions, it looks like the lidar system used by google's automated car system has decent resolution out to at least 30m picking out mobile bodies in the static background and then identifying it. So it should have plenty of time to brake and stop long before there was any risk to the pedestrian attempting to cross the street.

Skip to about 6:40 in the video to see a visual representation of the detection system.